Abstract
Sea clutters observed on the images of two weather radars of the Naze Weather Station and the R. V. Keifu-maru are analysed to use them in determining the sea surface winds. Their usefulness is established but some problems to be solved are pointed out.
The observed intensity of the sea clutter is proportional to uγ, where u is the wind speed and γ the constant which was about twice the value observed on aircraft.
The observed directional distribution of sea clutters differs from the distribution in the case of incidence angles of 20°∼80° in that the former shows the strongest intensity in the upwind direction and the weakest in the downwind direction while the latter has one maximum intensity each in the upwind and the downwind direction and one minimum each in the two crosswind directions.
It is known that the sea clutter of EM waves is mostly due to the Bragg scattering in the range of incidence angle between 20° and 80° which is adopted in the air-borne and satellite-borne scatterometers. In the case of an incidence angle of about 90° the sea backscatter is also due to the Bragg scattering, but is strongly affected by coexisting wind waves and swells which make the shadowing of the sea surface and the modulation of capillary waves. Our results can be explained qualitatively by considering these effects, but many truth data on the sea surface conditions and controlled experiments are needed to obtain qualitative estimations.
